Fast-neutron nuclear reactors generally comprise a large vessel filled with a coolant fluid such as liquid sodium, in which the reactor core, consisting of adjoining assemblies, is immersed. The core unit consists of assemblies of different types which may be fuel assemblies, fertile assemblies, absorbent assemblies or neutron protection assemblies made of steel.
When the reactor is operating, the removal of the thermal power released or received by each of these assemblies must be ensured. This thermal power can vary essentially as a function of the type of the assembly, of its position in the core and of its residence time in the reactor during operation.
Cooling of the assemblies is ensured by a circulation, in an axial direction of these assemblies, of the sodium coolant filling the vessel. This sodium circulation generally takes place in the vertical direction and from the bottom upwards, the assemblies comprising a first end or foot which is engaged in a stationary structure of the reactor, called a core grid, which is responsible both for supporting the core and for the distribution of sodium into each of the assemblies placed vertically above the core grid. The core grid rests on a flooring which itself bears on the bottom of the nuclear reactor vessel. The primary pumps of the nuclear reactor are responsible for feeding the core grid with cooled liquid sodium after it has passed through heat exchangers.
Because of the wide disparity existing between the quantities of power to be removed by means of each of the core assemblies, devices for controlling the flow rate of the circulating sodium coolant must be provided and adapted to each of the core assemblies in order to obtain a sodium temperature which is as uniform as possible on leaving the core.
It is known to employ flow rate control devices called depression-generating systems, which are fastened in a channel for the circulation of sodium in an axial direction passing through the foot of the assembly. These depression-generating systems comprise elements restricting the circulation of sodium inside the circulation channel and are fastened irremovably to the foot of the assembly. It is also possible to fasten these depression-generating systems irremovably to the core grid of the reactor, in the housing of the foot of a corresponding assembly.
It has also been proposed to fasten certain depression-generating systems irremovably to the interior of a reactor core grid, in order to modify, if desired, by changing the depression-generating systems, their capacity for controlling the flow rate of sodium in the corresponding assemblies. These operations of changing the depression-generating systems require the use of exceptional means of intervention in the reactor and the use of complex and wholly unconventional procedures.
In the case where the depression-generating system is fastened irremovably to the foot of the assembly, the flow rate passing through the assembly and the average sodium outlet temperature are calculated for a predetermined constant power. When the thermal power of the assembly varies in the reactor during operation, for example as a function of its residence time, when a fertile assembly is involved, or else as a function of the position which it occupies in the core, in the case of a fuel assembly, the constant and predetermined flow rate passing through the assembly, which is adapted either to the initial conditions or to the final conditions in which the assembly operates, necessarily introduces a thermohydraulic penalty, and hence a loss in efficiency, in the course of operation.
In the case where the depression-generating system is fastened irremovably to the core grid of the reactor, there is a risk of blocking of the corresponding channel and hence a risk of a complete loss of cooling of a fuel assembly. In certain cases, a blockage of this kind cannot be detected, with the result that remedial action cannot be taken promptly enough to be effective.
It therefore appears useful to have available a process making it possible to modify the control of flow rate in a fuel or fertile core assembly of a nuclear reactor and capable of being used in a simple manner and without involving exceptional means.
For example, in the case of the transfer of a fuel assembly from the center of the core to the periphery, into the region of the steel reflector assemblies, a 10-fold reduction in the flow rate of coolant fluid must be obtained to avoid any thermohydraulic penalty.
A flow rate reduction of this kind is necessary, in particular, if it is desired to effect the storage of the fuel assemblies inside the reactor vessel.
In the case of a fertile assembly whose power increases with the residence time in the reactor core, a thermohydraulic penalty due to the overcooling of the assembly must be accepted at the beginning of its life.
The core assemblies of a fast-neutron nuclear reactor comprise, at the end opposite from their foot, in the axial direction, an end part called a head, which comprises a central bore in which a housing is provided for hooking on the claws of an assembly manipulating grab. This central bore is extended in the direction of the foot by a coolant fluid passage channel in axial direction, whose diameter is smaller than the diameter of the bore in its upper part allowing the assembly to be grabbed.
The head part of the fuel or fertile assemblies consists of a massive steel component responsible for the upper neutron protection of the assembly.
This head part of the assembly has so far never been employed to receive a device for controlling the flow rate of coolant fluid, such as a depression-generating system.